Upgrade setup-flyctl action from Node.js 20 to Node.js 24

[artificialrobot]

Context

GitHub is deprecating Node.js 20 for GitHub Actions runners. Actions will be forced to run with Node.js 24 by default starting June 2, 2026.

Reference: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Current state

setup-flyctl/action.yml on master currently specifies using: node20. The latest release (1.5) also uses node20.

Request

Update the action to use node24 (or node22 as a stepping stone) so consumers don't hit breakage when the forced cutover happens.

Workaround

Users can set FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true to test compatibility early, but the action itself needs to be updated to avoid the deprecation warning:

Node.js 20 actions are deprecated. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026.

This follows the same pattern as the previous upgrade in #50 / PR #51.

[giacomorebonato]

How do people usually deploy to Fly.io without this action?

[p-j]

it's addressed by #108